1. Field of the Invention
This invention relates to an epitaxial structure, more particularly to an epitaxial structure having a low defect density and usable as a substrate for forming a semiconductor device thereon.
2. Description of the Related Art
In gallium nitride based light-emitting devices, a sapphire substrate or a silicon carbide substrate is usually used for growth of gallium nitride-based layers thereon. During growth of the gallium nitride-based semiconductor layers on the substrate, dislocations due to the lattice mismatching are formed and propagate into the active layer of the light-emitting device, thereby reducing light emitting efficiency thereof.
U.S. Pat. Nos. 6,051,849 and 6,608,327B1 disclose a semiconductor structure including a base layer on which a buffer layer, a first epitaxial layer and a patterned silicon dioxide layer are sequentially stacked. Subsequently, a second epitaxial layer is formed on the first epitaxial layer and the patterned silicon dioxide layer using lateral epitaxial overgrowth techniques. By covering portions of the first epitaxial layer with the patterned silicon oxide layer, defects or dislocations in the second epitaxial layer, which propagate from the first epitaxial layer, can be reduced. Therefore, dislocations formed in the active layer can also be reduced, which results in improvement of light emitting efficiency of the light-emitting device when formed on the semiconductor structure.
However, an increase in epitaxial layers may cause a decrease in yield. In addition, when the number of the epitaxial layer increases, the reduction of the defects attenuates, which fails to effectively improve quantum efficiency of the light-emitting device.
U.S. Patent Application Publication No. 2008/0068291A1 discloses a semiconductor structure including a patterned base layer formed with a plurality of recesses, and a first epitaxial layer formed on the patterned base layer. Subsequently, a patterned mask is stacked on the first epitaxial layer in positions corresponding to recess-free regions for preventing defect propagation therethrough.
U.S. Pat. No. 7,364,805B2 discloses a method of forming spontaneously a plurality of recesses in positions corresponding to dislocations by etching without patterning by a mask. Referring FIGS. 1 to 3, an epitaxial substrate 1 includes a base layer 11, a first epitaxial layer 12, and a coated film 13. The first epitaxial layer 12 has an epitaxial surface 122, a plurality of defects 123, and a plurality of etch pits 121 formed corresponding to the defects 123. The coated film 13 is formed on the epitaxial surface 122 and in the etch pits 121. Then, the coated film 13 formed on the epitaxial surface 122 is removed using reactive ion etching (RIE) techniques for subsequent growth of epitaxial layers thereon. However, defects are prone to increase during removal of the coated film 13 using reactive ion etching techniques. In addition, the etch pits 121 are formed on top ends of the defects including screw dislocations and edge dislocations due to etching by heat and gas.